1. Field of the Invention
The invention relates to a radiofrequency transmitter and/or receiver. More particularly, the invention relates to satellite broadcasting devices.
2. Description of the Prior Art
Satellite broadcasting has many advantages over radio transmission called “terrestrial” broadcasting. Among the advantages, mention may be made of the direct visibility between transmitter and receiver, the absence of echoes and, above all, a broad band of frequencies that can be used.
The success of satellite broadcasting has had the effect of gradually saturating the spectrum of available frequencies. It is necessary to use ever higher frequencies with ever broader bandwidths. At the present time, a satellite broadcasts over several carriers lying within the same frequency band. As an example, a satellite television receiver may, for example, receive 20 channels lying between 11.7 GHz and 12.1 GHz.
FIG. 1 shows an example of a satellite receiver of a conventional type which comprises a receiving block labelled LNB (Low Noise Block), mounted for example at the focus of a parabolic dish, and an internal unit labelled TUNER. The LNB receiving block comprises an antenna 1 followed by a low-noise amplifier 2. The signal delivered by the amplifier 2 is transposed to an intermediate frequency by means of a mixer 3 and an oscillator 4. The signal transmitted from the LNB receiving block to the TUNER internal unit has a working bandwidth lying around 1 to 2 GHz.
The TUNER internal unit comprises a first band-pass filter 5 which lets through only the signal of the working bandwidth coming from the LNB block. A frequency synthesizer 6, consisting for example of a voltage-controlled oscillator 7 and a phase-locked loop 8, delivers a tuning signal which allows a mixer 9 to frequency-transpose the working bandwidth so that a channel selected from this bandwidth lies around a predefined intermediate frequency. A second, more selective, filter 10 removes the other channels present in the working bandwidth. A mixer 11 coupled to a local oscillator 12 transposes the selected channel from the intermediate frequency to the baseband.
To achieve high-datarate transmission, the channels may be less broad than for television image broadcasting, that is to say between 5 and 50 MHz, but must be more numerous as the information is customized by each user. If one is in the Ka band, the frequency assignment made by the various standardization organizations defines the various frequencies that can be used, which are sometimes non-contiguous.
To obtain the desired bandwidth, it may be necessary to use non-contiguous bands in order to have a very broad bandwidth. As an example, it is possible to have a band consisting of two sub-bands, for example between 18.3 and 18.8 GHz and between 19.7 and 20.2 GHz separated by a forbidden band 900 MHz in width. The working bandwidth is then spread out over 1.9 GHz.
The use of a conventional device such as that in FIG. 1 is not possible for many reasons. Among others, the frequency synthesizer 6 would have to operate over a 1.9 GHz range. Unfortunately, it is very difficult to produce such a synthesizer using current means. The problem is solved for satellite television receivers by using several LNB blocks which bring the various bands down to a single intermediate frequency range or by using several TUNER units which operate at various frequency ranges.